The obligate intracellular bacteria belonging to Rickettsia species are etiological agents of the spotted fever and typhus groups (SFG and TG) of rickettsial diseases in almost all geographic locations worldwide. Rocky Mountain spotted fever and epidemic typhus, caused respectively by R. rickettsii and R. prowazekii, are serious infections that can be fatal in children, elderly, and immunodeficient or misdiagnosed/untreated patients. A major clinical hallmark is the infection of endothelial cell lining of vessels, ultimately leading to vascular inflammation and damage. While apoptosis serves as an important host defense mechanism to restrict proliferation and ensuing pathogenesis during bacterial infections, intracellular rickettsiae require intimate association with the target host cells for their growth/spread. It is thus possible that like other intracellular pathogens, rickettsiae may also utilize strategies to inhibit host apoptosis early during the infection for sake of their own survival. Identified and described by our laboratory, one such strategy during endothelial cell infection of R. rickettsii is the activation of NF-?B, but nothing is known about cell signaling events during infection with and potential anti-apoptotic activities of typhus group organisms. To address this critical gap in the existing knowledge of rickettsial pathogenetic mechanisms is highly significant because of distinctive differences in the intracellular behavior of SFG and TG organisms and the potential for intentional use of R. prowazekii (Class B on CDC list) and R. rickettsii (Category C) as bioterror agents. The objectives of this exploratory grant application are to investigate the hypothesis that early during the infection, rickettsiae trigger anti-apoptotic mechanism(s) in host cells to facilitate the establishment and progression of infection and to define potential similarities or differences in host signaling mechanisms exploited by SFG and TG rickettsiae by conducting simultaneous and thorough comparative analyses. Aim 1 is designed to identify apoptotic and anti-apoptotic signaling pathways induced by representative Rickettsia species and will focus on 'shift(s)' in the equilibrium between pro- and anti-apoptotic genes and proteins of Bcl-2, IAP, and p53 families. Aim 2 will further decipher the complex mechanistic aspects of apoptosis regulation during Rickettsia infection by discerning the involvement of extrinsic or intrinsic cascades and identifying the upstream initiator and downstream effector caspases. Employing a combination of conventional biochemical and state of the art microscopic/molecular genetic approaches, we will reveal novel insights into the molecular basis of Rickettsiae-mediated modulation of apoptosis and improve our understanding of the unique perspective of manipulation of host functions by parasitic intracellular pathogens. Acquisition of this new knowledge regarding adaptations in host cell signaling and recent progress in the genomic and proteomic characterization of Rickettsia organisms will enable us to identity and isolate potential rickettsial anti-apoptotic activity(ies), which can be targeted to develop new and improved strategies aimed at enhancing clearance of infected cells early to avoid disseminated endothelial infection and associated vascular pathologies. [unreadable] [unreadable] [unreadable] [unreadable]